Recurrent spontaneous pregnancy loss is an important women's health issue. The frequency of clinically recognized spontaneous abortion in the general population has been estimated to range between 15 and 20 percent of all recognized conceptions. Approximately 50 percent of spontaneously aborted fetuses are karyotypically abnormal, and half have normal chromosome studies. Women who have unexplained recurrent spontaneous abortion are frequent, and an underlying genetic basis for recurrent pregnancy loss is frequently assumed. However, investigations into possible genetic causes of cytogenetically normal recurrent spontaneous abortion have been rare or non-existent, and no responsible genes or gene products identified. The lack of genetic studies is likely due to the high incidence of spontaneous abortion in the general population, and difficulty in establishing investigative protocols. Here we report a 53 member pedigree where many family members have recurrent spontaneous abortion in the absence of any other clinical symptoms or phenotype. We show a statistically significant association between the phenotype of recurrent spontaneous abortion and highly skewed X inactivation patterns, and have genetically mapped the locus responsible for this "molecular" phenotype to Xq28. Thus, we have identified the first genetic locus for recurrent spontaneous abortion, and have shown that the inheritance mechanism is "X-linked lethal". The hypothesis that our proposed research will test is that a subset of females that show recurrent spontaneous abortions are carriers for X- linked lethal traits. Our first specific aim is to define the molecular basis of the phenotype in the Xq28-linked family through positional cloning techniques. The second specific aim is to ascertain and collect women with recurrent cytogenetically normal pregnancy loss, and two control groups. These women will be studied for X inactivation patterns to determine the extent of association between recurrent spontaneous abortion and the carrier state for X-linked lethal traits. While there are likely to be many different causes for recurrent pregnancy loss, the proposed research will begin to dissect the molecular basis for the subset that are genetically determined as mendelian traits. This proposal represents a synergistic collaboration between a laboratory well versed in molecular analysis of inherited traits, and clinicians and epidemiologists with considerable expertise in women's health issues.